Magnetically activated well tool

ABSTRACT

A detector assembly and methods including a magnetoresistive sensor capable of detecting anomalies in the wall of a casing string disposed in a wellbore. Examples of anomalies include gaps between casings such as due to casing joints, air gaps in casing threads such as due to flush casing joints, enlarged casing wall thickness such as due to external casing collars, damaged casing, perforations, and other discontinuities or deformities in the casing. The detector assembly and methods detect perturbations in the earth&#39;s magnetic field caused by the anomalies. The detector assembly generates essentially no magnetic or electromagnetic field so that other downhole instrumentation is not affected by its presence.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is related to the following applications:U.S. application Ser. No. 09/286,362, filed Apr. 5, 1999 and entitledMagnetically Activated Well Tool, hereby incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] Not Applicable.

BACKGROUND OF THE INVENTION

[0003] 1. Field of the Invention

[0004] The present invention relates to locators for locating anomaliesin a casing string for a wellbore such as casing joints. Moreparticularly, the invention relates to apparatus and methods fordetecting, identifying, and locating anomalies in strings of tubularmembers by sensing the natural magnetic fields induced within thestring, such as perturbations in the natural magnetic fields due tofringe effects caused by the anomalies.

[0005] 2. Description of the Related Art

[0006] Casing collar locators are used to locate joints within theborehole casing. The locator is suspended on a wireline cable and passedthrough the cased borehole. The locator detects the collars used atjoints in the casing string as the locator is moved upwardly and/ordownwardly through the casing. Various types of casing joints are usedto connect adjacent ends of the casing section in a threaded engagement,such as upset joints and exterior collar joints. As the locator movesadjacent to a casing joint, it detects a change in the magnetic readingsresulting from the change in casing thickness, or change in mass ofmetal associated with the casing wall or it detects a change in thepolarity of adjacent sections of casing.

[0007] Casing collar locators are extremely important tools for downholeoperations. They are required for depth correction operations and forthe accurate placement of downhole tools, such as anchors, bridges,whipstocks, profiles, and packers. For example, it is desired to avoidsetting a downhole tool on a casing joint since the joint presents a gapor discontinuity in the casing wall that may prevent the downhole toolfrom sealing or anchoring properly.

[0008] In order to detect a casing joint, conventional casing collarlocators typically rely on the generation of a relatively powerfulmagnetic field from the locator using either a permanent magnet or bypassing a current through a coil to induce magnetism. A significantamount of power is required to generate the magnetic field. As the coilpasses adjacent a casing joint, the flux density of the magnetic fieldis changed by the variation in the thickness of metal provided by thejoint. The change causes an electrical output signal to be generatedthat indicates the presence of the casing joint, and this output signalis transmitted to the surface of the well through a wireline.

[0009] Unfortunately, conventional casing collar locators suffer fromoperational disadvantages and limitations of their effectiveness.Conventional locators are not greatly sensitive, in general, todiscontinuities, anomalies, or other changes in the wall of the casingbecause prior art locators are necessarily large and often severalinches to a few feet in length. This causes the locators to have a largeresolution such that they cannot detect changes in the magnetic fieldsof the casing that are less in length than the locator. Thus, such priorart locators are insensitive to small anomalies in the casing.

[0010] As a result of not having a high resolution, conventional casingcollar locators are reliable only in a “dynamic” mode wherein thelocator is moved rapidly through the wellbore casing in order toaccurately detect the presence of casing joints. If the locator is movedtoo slowly, the changes in the signal indicative of the presence of acasing joint, such as a collar, may be too gradual to be recognized bythe well operator. Dynamic location of casing joints thus isdisadvantageous because it tends to provide less accurate real-timeinformation concerning the position of the casing joint. For example, ifit is desired to set a packer five feet below a particular casing jointin a wellbore, a conventional casing collar locator would be movedrapidly either upwardly or downwardly through the wellbore until theparticular casing joint is detected. When that occurs, a signal isprovided to the wellbore operator which indicates the location of thejoint. Due to movement of the locator through the casing, however, thecasing collar locator is no longer positioned proximate the casing jointby the time the operator receives the signal and reacts to it bystopping movement of the locator. The precise position of the casingjoint must then be somewhat approximated given the current position ofthe locator within the wellbore.

[0011] Additionally, conventional locators locate casing joints bydetecting a difference in thickness of the casing wall such as thepresence of an external upset or collar. These devices are actually,“collar” locators rather than “joint” locators. As a result, they areunable to reliably detect a “flush” joint where the casing wallthickness is not appreciably altered by the presence of the joint. Ajoint is considered flush where the adjacent casing sections arethreaded directly to one another or where the upset or collar isunusually thin or contains very little metal.

[0012] In addition, because conventional casing collar locators generatea significant magnetic field, they tend to interfere with other downholeinstrumentation that rely upon accurate magnetic readings. For example,a compass-type magnetometer that is attempting to find magnetic northcan be confused by the magnetic field generated by the casing collarlocator. Some induction-type locators are known that generate andtransmit strong electromagnetic waves, rather than magnetic fields, todetect casing joints. Unfortunately, these devices also tend tointerfere with downhole instrumentation.

[0013] A need exists for a locator that can more reliably detect thepresence of casing section joints in a wellbore and particularly flushjoints that do not employ radially enlarged upsets or collars. Further,a need exists for a locator that generates a minimal or no magneticfield that affects the operation of other downhole instrumentation.

[0014] In addition, a need exists for a detector that can detect,identify, and/or locate anomalies, such as deformities, discontinuities,perforations and the like, in a cased borehole. To locate the depth andangular orientation of a perforation, for example, requires a verysensitive locator because of the small size of the perforation. Theperforation generally is less than one inch in diameter and typicallyonly one-fourth inch in diameter, thus providing a very small change inthe continuity of the casing wall and requiring a very sensitivelocator.

[0015] By way of background, to complete a well, the cased borehole isperforated adjacent the formation to be produced. A perforating trip ismade by lowering into the well bore a perforation tool mounted on thelower end of a wireline or tubular work string. The perforation tool or“gun” assembly is then detonated to create a series of spacedperforations extending outwardly through the well casing, the cementholding the casing in place in the wellbore, and into the productionzone. Although these perforations may have a random pattern, typicallythe perforations are made in a spiral pattern around the casing string.

[0016] Often the well is treated to enhance production. Well treatmentmay include treating the formation with chemicals, “fracturing” or a“fracing” the formation, injection of high pressure fluids, acidizing,jetting, or pumping proppant into the formation to maintain thefractures in the formation. The well is treated or stimulated by pumpingfluids through the perforations and into the formation. For example,during fracing, a tubular discharge member having a series of spaceddischarge ports is lowered into the well on a work string. Packers areset above and below the perforations to form an isolated region. Thedischarge ports are preferably aligned with the perforations. A slurryis then pumped down the workstring and discharged through the ports inthe discharge member causing the slurry to flow through the perforationsand into the surrounding production zone. The slurry may includeproppant or other treatment fluid.

[0017] Well treatment techniques have several well known problems,limitations, and disadvantages. For example, when the discharge memberis lowered into the well bore, it is difficult to obtain a precisealignment (in both the axial and angular directions) between thedischarge ports in the discharge member and the perforations in thecasing. The usual result is that some degree of misalignment existsbetween the discharge ports and the perforations. When the ports andperforations are not in alignment, the high pressure fluid must follow atortuous path before entering the perforations after it is dischargedfrom the discharge member. Because the treatment fluid is discharged ata very high pressure and often is highly abrasive, this tortuous flowpath can cause severe abrasion and wear problems in the casing.

[0018] In addition, it is important that the packer or packers not beset in the perforated region of the casing. If a packer is set in thearea having the perforations, the fluid flowing out of discharge portsand through the perforations into the formation may flow back into thewellbore annulus through perforations that are above or below thatportion of the wellbore annulus that is isolated by the packers.Turbulence caused by the high pressure and abrasive fluid flowing backinto the annulus creates a pressure differential across the packers andtends to erode or “wash out” and ruin the packers. Additionally, it isimportant that the packer or packers not be set within a casing joint,but instead be set in blank pipe. Typically, there are gaps between thealigned ends of casing sections at the casing joints. If the packer isset in this region, then the packer will not seal properly and holdpressure to isolate the intended interval. When this occurs, thetreatment fluid can pass out of the interval and into the annulus andwash out and erode the packer. Accordingly, it is critical to know thelocation of the perforations and the casing joints to ensure that thepacker is not set within the perforations or within a casing joint.Unfortunately, properly positioning the packer with respect to theperforations and casing joints has been difficult to achieve.

[0019] Furthermore, even if the depth of the discharge member isprecisely known, there still exist problems that are introduced due toinaccuracies in determining the actual depth of the perforations. Asstated above, the step of perforating the well typically includesrecording the depth and location of the perforations; however, usingperforation equipment with both wireline and tubing nevertheless doesnot always provide accurate depth measurements, due again to thetendency of the tubing or wireline to expand with down hole temperaturesor to bend in the borehole.

[0020] A need thus exists for a detector that can more reliably detectthe presence of anomalies, such as perforations, in the cased borehole.Further, a need exists for a detector that generates a minimal or nomagnetic field that would affect the operation of other downholeinstrumentation.

[0021] Giant magnetoresistive or GMR magnetic field sensors are know foruse in high accuracy compasses and geophysical applications such asmagnetic field anomaly detection in the earth's crust. GMR sensors areconstructed from alternating, ultrathin layers of magnetic andnon-magnetic materials. GMR sensors provide high sensitivity to changesin a nearby or surrounding magnetic field. GMR sensors of this type aredescribed in the prior art NVE brochure entitled “NVE—NonvolatileElectronics, Inc. The GMR Specialists” with errata sheets, and arecurrently manufactured and marketed by Nonvolatile Electronics, Inc.,11409 Valley View Road, Eden Prairie, Minn. 55344-3617, (612) 829-9217.The GMR sensor uses a “giant magnetoresistive effect” to detect a changein electrical resistance that occurs when stacked layers offerromagnetic and non-magnetic materials are exposed to a magneticfield.

[0022] The present invention overcomes the deficiencies in the priorart.

SUMMARY OF THE INVENTION

[0023] The present invention provides an apparatus and methods forreliably detecting, identifying, and locating anomalies in a casingstring extending into a wellbore. The apparatus and methods include adetector assembly with a sensor that can sense casing anomalies thatvary the magnitude of the magnetic field or that have fringe effectsthat cause perturbations, or changes, in magnetic fields that areinduced in the casing sections by the earth's natural magnetic field.

[0024] To detect casing joints, the induced magnetic fields includeattractive forces that result from magnetic fringe effects proximate thelongitudinal ends of the casing sections. The attractive forces arepresent at the connective joints of the casing string, thus presentingperturbations in the magnetic fields associated with the casing. Theinventive methods and apparatus will detect voids, such as gaps anddiscontinuities, associated with a casing joint as well as an increasedthickness caused by an upset or external collar associated with a casingjoint. Thus, the inventive methods and apparatus are capable ofdetecting flush joints as well as other conventional casing joints.

[0025] The apparatus and methods also provide clear and reliable signalsindicative of the presence of small anomalies including perforationswhich are small in size and flush joints where there is no appreciablechange in the diameter of the casing at the joint. As a result, thepossibility of a well operator failing to recognize such a signal isminimized.

[0026] The detector assembly of the present invention generatesessentially no magnetic or electromagnetic field. As a result, thepresence of the detector does not affect other downhole instrumentation.The detector assembly relies upon the earth's natural magnetic field topolarize and thus induce a magnetic field in the surrounding casingsections. The detector assembly detects perturbations in thisnaturally-induced magnetic field, such as will result from the fringeeffects associated with anomalies, such as gaps, holes, ordiscontinuities in the casing wall. The detector assembly also easilydetects the magnetic signature associated with the presence of asurrounding casing collar.

[0027] Further, methods and apparatus of the present invention providefor accurate measurement of lengths and distances, such as the length ofcasing joints or the distance between such joints.

[0028] The present invention also provides a detector assembly with asensor having a very small physical size and that uses very littlepower. Further, the detector assembly of the present invention does notneed to be moved rapidly through the wellbore in order to reliablydetect an anomaly in the casing string. Thus, methods are described for“static” detection of casing anomalies where the detector assembly ismoved either very slowly or not at all and that detector can stillreliably detect the casing joint.

[0029] The detector assembly of the present invention can also locatesmall anomalies or changes in the thickness of the wall of the casingstring. For example, the detector assembly can detect, identify, andlocate perforations in the casing having a one-quarter inch diameter.Only one sensor is used in the detector assembly for perforationpatterns with perforations on only one side of the casing in a givenplane perpendicular to the longitudinal axis of the casing. Forperforation patterns with perforations on more than one side per plane,more than one sensor is used in the detector assembly to detect theindividual perforations. However, with perforation patterns havingperforations on more than one side per plane, one sensor can be used todetect the perforation zone of the casing. The detector assembly alsolocates the perforations by determining the depth and angularorientation of the perforations for setting one or more packers in thecasing string.

[0030] Thus, the preferred and alternative embodiments comprise acombination of features and advantages that enable them to overcomevarious problems of prior art devices. The various characteristicsdescribed above, as well as other features, will be readily apparent tothose skilled in the art upon reading the following detailed descriptionof the preferred and alternative embodiments, and by referring to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0031] For a more detailed description of the preferred and alternativeembodiments, reference will now be made to the following accompanyingdrawings:

[0032]FIG. 1 is a cross-section through the casing string illustratingvarious examples of anomalies which may be detected, identified andlocated by the detector assembly of the present invention;

[0033] FIGS. 2-4 are cutaway side views of a pair of casing sectionsjoined to one another by a flush joint and containing an exemplarylocator for casing joints constructed in accordance with the presentinvention;

[0034]FIG. 5 is an enlarged view of a portion of a flush casing joint;

[0035]FIG. 6 is a cutaway side view of a pair of casing sections joinedby an external collar connection and containing an exemplary locator forcasing joints constructed in accordance with the present invention;

[0036]FIG. 7 illustrates the induction of magnetic forces in a pair ofcasing sections;

[0037]FIG. 8A is a schematic of a sensor without an external magneticfield and FIG. 8B is a schematic of a sensor with an external magneticfield;

[0038]FIG. 9 is a schematic diagram illustrating exemplary signalsreceived and generated by the signal processor;

[0039]FIG. 10 is a cutaway side view of a casing section withperforations containing an exemplary locator constructed in accordancewith the present invention;

[0040]FIG. 11 is an elevation view of a perforated casing havingperforations in a spiral pattern;

[0041]FIG. 12 is a cross section view of a perforated casing havingperforations which are opposed to each other; and

[0042]FIG. 13 is a cross-sectional elevation view of the lower end of acased borehole with an earthen borehole extending below the terminal endof the casing.

DETAILED DESCRIPTION OF THE PREFERRED AND ALTERNATIVE EMBODIMENTS

[0043] The apparatus and methods of the present invention relategenerally to detecting, identifying, and locating anomalies in the wallof a string of tubular members by sensing changes or perturbations innatural magnetic fields induced within the string. The present inventionis not limited to any particular type of anomalies and in particular isnot limited to casing joints and perforations which are described asexamples of the application of the apparatus and methods of the presentinvention. The drawings and the description below disclose in detailspecific embodiments of the present invention with the understandingthat this disclosure is to be considered an exemplification of theprinciples of the invention, and is not intended to limit the inventionto that illustrated and described in the disclosure. Further, it is tobe fully recognized that the different teachings of the embodimentsdiscussed below may be employed separately or in any suitablecombination to produce desired results.

[0044] Referring initially to FIG. 1, a detector assembly 66 includesone or more sensors 70 for detecting, identifying, and locatinganomalies in the steel or metal casing 14 making up the casing string 11disposed in the borehole of a well. An anomaly is defined as anyvariance in the thickness T in the wall 21 forming bore 16 including theabsence of thickness, such as a hole through the casing wall. ThicknessT is defined as the uniform thickness of the blank portion 15 of thetubular member making up a casing section between the threaded endconnections or joints.

[0045]FIG. 1 illustrates various examples of anomalies which may bedetected, identified, and/or located by detector assembly 66. It shouldbe appreciated that the anomalies shown in FIG. 1 are only illustrativeof anomalies in a casing string and should not be considered limiting ofthe anomalies which may be detected, identified and/or located bydetector assembly 66. FIG. 1 illustrates various anomalies including ahole 112 passing through the wall 21 of casing string 11, an internalprofile 114 for locating a well tool or tubular member for performing awell operation within the well, a well reference member 116 permanentlydisposed within a casing section downhole for locating a well tool ortubular member downhole, a casing collar connection joint 118 increasingthe thickness T of the casing string 11, a upset casing joint 120forming an annular gap in the interior of casing string 11, anothercasing joint 122 forming an external angular gap, damage 124 to thecasing, a flush joint 126 having air gaps in the threads, and a scar 128on the exterior wall of casing string 11. All of the above are examplesof anomalies. It should be appreciated that some anomalies relate tovariations in the thickness T of wall 21, others include a reduction ofthe mass at a particular point in the wall 21 of casing string 11 andothers include an aperture, hole, or perforation extending completelythrough the wall 21 of casing string 11. It also can be seen thatanomalies may include annular anomalies which extend around the internalor external circumference of wall 21, others include air pockets or gapsinterior to wall 21, and other anomalies include a reduction in theinterior or exterior mass of the wall 21 due to scarring or other damageto the wall surface.

[0046] It should be appreciated that detector assembly 66 may be used todetect an anomaly or may be used to identify an anomaly, or may be usedto locate an anomaly. Detector assembly 66 may also be used to measurethe dimensions of an anomaly. In locating an anomaly, detector assembly66 may determine the depth of the anomaly, depth being the distancebetween the anomaly and the surface measured through the bore of casingstring 11, and may determine the angular orientation of the anomalywithin the cylindrical wall 21 of casing string 11. In a vertical casingstring, the angular orientation will be the azimuth of the anomaly.

[0047] The sensor 70 senses an increase or decrease in the mass of thewall 21 at a particular point along casing string 11 as well as sensesthe absence of mass. Anomalies which include breaks in the interiorsurface of wall 21 form fringe effects which cause perturbations in thenaturally induced magnetic field of the wall 21 of casing string 11. Thevariation in mass and/or the fringe effects alter the external magneticfield around sensor 70 causing an increase or decrease in the resistanceof sensor 70 thereby altering the flow of current through sensor 70. Asignal is generated by the change in current flow and the signal istransmitted to the surface to provide a detection, identification, orlocation of the anomaly in casing string 11.

[0048] Referring now to FIGS. 2-5, there is shown an example of usingthe detector assembly 66 for detecting, identifying and locating acasing joint in the casing string. A borehole section 10 is depictedextending through a formation 12 in the earth. The borehole section 10includes a string 11 of steel or metal tubular casing 14 forming acylindrical wall 21 that encloses and defines bore 16 therethrough.Cement 18 surrounds the radial exterior of the casing 14.

[0049] A plurality of elongated tubular casing sections makes up thestring of casing 14. Two representative casing sections 20, 22 are shownaffixed to one another at a threaded joint 24 that is shown in FIGS. 2-4and in a closer view in FIG. 5. The joint 24 is made up of a pin-typeconnector 26 on the upper casing section 20, which is secured within acomplimentary box-type connector 28 on the lower casing section 22. Theparticular joint depicted in FIGS. 2-5 is a flush joint wherein there islittle or no change in the thickness T of the casing 14 at the joint. Asis apparent, there is no external collar used to join the two casingsections.

[0050] The threads of the joint 24 include a plurality of air gaps 30,best shown in FIG. 5, that are inherent in any such threaded connectionwhere the generally complimentary threads of the two sections 20, 22 areinterleaved. Further, discontinuities in the form of annular gaps 32, 34are present at either end of the threaded joint 24. Gaps 32, 34 areformed between the terminal end of the casing section 20, 22 and theshoulders 25, 27 at the base of the threads on sections 20, 22,respectively.

[0051]FIG. 6 depicts a more conventional casing collar joint 36 in whichthe pair of casing sections, designated as 20′ and 22′, areinterconnected by a threaded collar 38 that is used to secure a pair ofpin-type connectors 40. A discontinuity in the form of annular gap 42 ispresent between the terminal ends of the two adjacent casing sections20′ and 22′.

[0052] The earth's natural magnetic field causes metallic casingsections to act as magnetic dipoles, thus providing their own naturallyinduced magnetic fields. Referring for the moment to FIG. 7,illustrative magnetic lines of force 50 are depicted around casingsections 20, 22. The magnetic lines of force 50 show a magnetic fieldthat is induced within the casing sections 20, 22 by the earths naturalmagnetic field 52, or the magnetic forces travelling from the magneticnorth to south poles of the earth. As a practical matter, the inducedmagnetic field 52 is very weak, but it is capable of being detected bysuitably sensitive instrumentation. In essence, the natural field 52polarizes each section 20, 22 to act as a dipole, providing attractivemagnetic forces 50 running from their north poles 54 to their southpoles 56. Each casing section 20, 22 is polarized in a common directionso that their north and south poles 54, 56 are commonly oriented. Inaddition, it should be understood that when the casing sections areinterconnected, the entire casing string thus formed will act as asingle dipole to some extent as well.

[0053] It is well known that the magnetic field is stronger proximatethe north and south ends of a dipole. When the casing sections 20, 22are placed close to one another in an end-to-end relation, as depictedin FIG. 7, there are attractive magnetic end effects, or “fringeeffects,” 58 that act between the two adjacent casing sections 20, 22.When the two casing sections 20, 22 are joined to one another via athreaded connection 24 (or 36), the fringe effects 58 continue toprovide lines of attractive magnetic force between portions of theinterconnected casing sections at and around the connection point. Theselines of attractive force generally correspond to the presence of smallgaps or separations, such as at 30, between the two sections. By way ofexample, FIG. 5 is a close up view of a portion of the threaded flushjoint connection 24 showing illustrative lines for these attractiveforces 60 located at the gaps 26 and the discontinuities 28 and 29 forthe connection. The aggregate of these small attractive forces 60 leadsto an increased fringe effect magnetic signature 62 that is depicted bymagnetic force lines in FIGS. 2-4.

[0054] An increase localized magnetic signature 63 is also shown to beassociated with the collar joint 36 in FIG. 6. This signature 63 resultsfrom the increase mass of metal provided by the external collar 38 aswell as the attractive magnetic effects associated with thediscontinuity 42 in the wall of the casing string.

[0055] FIGS. 2-4 and 6 also show suspended within the bore 16 of thestring of casing 14, a wireline 64 that is disposed into the wellbore 16from the surface (not shown). The wireline 64 is adapted to transmitpower and data in the form of a modulated electrical signal. It ispreferred that the wireline 64 include power and ground wires, datatransmission lines, and command/response transmission lines. Thewireline 64 also supports detector assembly 66 that includes a pressurebarrel 68 constructed of a non-magnetic material such as berylliumcopper. The pressure barrel 68 is constructed to be resistant to fluidsand capable of withstanding downhole pressures without collapsing. Itshould be appreciated that detector assembly 66 may be suspended ontubing rather than a wireline.

[0056] The sensor 70 in detector assembly 66 may be a “giantmagnetoresistive,” or GMR magnetic field sensor that is housed withinthe pressure barrel 68. GMR sensors are constructed from alternating,ultrathin layers of magnetic and non-magnetic materials. GMR sensorsprovide high sensitivity to changes in a nearby or surrounding magneticfield. GMR sensors of this type are currently manufactured and marketedby Nonvolatile Electronics, Inc., 11409 Valley View Road, Eden Prairie,Minn. 55344-3617, (612) 829-9217. The GMR sensor is adapted to detect achange in a surrounding magnetic field and, in response thereto,generate a signal indicative of the change. The sensitivity of the GMRsensor permits detection of small anomalies in the surrounding magneticstructure, such as the gaps 30 and the discontinuities 32, 34 of thecasing joint 24. As a result, joints between a pair of interconnectedcasing sections can be detected by the detector assembly 66. It is notedthat a GMR sensor itself generates essentially no magnetic signatureand, therefore, will not affect the operation of other downholeequipment that detect or rely upon magnetic readings.

[0057] Referring now to FIGS. 8A and B, there is shown a schematic of a“giant magnetoresistive,” or GMR magnetic field sensor 70 described inthe prior art NVE brochure entitled “NVE—Nonvolatile Electronics, Inc.The GMR Specialists” with errata sheets, all hereby incorporated hereinby reference. The “giant magnetoresistive effect” is a change inelectrical resistance that occurs when stacked layers of ferromagneticand non-magnetic materials are exposed to a magnetic field. Highsensitivity low field GMR materials are to be used in high accuracycompasses and geophysical applications such as magnetic field anomalydetection in the earth's crust.

[0058]FIG. 8A illustrates sensor 70 with no external magnetic field andFIG. 8B illustrates sensor 70 with an external magnetic field. Sensor 70includes alternating layers of magnetic and nonmagnetic materials. In atypical sensor 70, there are two layers of magnetic material 90, 92,such as an alloy, which are separated by an interlayer of a conductive,nonmagnetic material 94. A magnetic field 100 applied to sensor 70induces a current 102 to flow through materials 90, 92, 94 which providea resistance. The resistance to current 102 is high. Referring to FIG.8A and as shown by arrows 96, 98, the magnetic moments in magneticmaterials 90, 92 face opposite directions due to magnetic field 100.Referring to FIG. 8B, applying an external magnetic field 104 causes themagnetic moments 96, 98 to line up in the direction of the current 102from magnetic field 100. Electrical resistance drops dramatically. Asthe external magnetic field 104 varies, the current varies.

[0059] The sensor 70 is very small having typical dimensions of 0.154inches by 0.193 inches by 0.054 inches. Thus, sensor 70 is sufficientlysensitive to detect perturbations of a similar size, i.e., substantiallyless than an inch. The advantages of the GMR sensor include reducedsize, high signal level, high sensitivity, high temperature stability,and low power consumption.

[0060] The detector assembly 66 also includes a signal processor 72 thatis operably interconnected with the sensor 70. The signal processor 72receives the signal provided by the sensor 70, amplifies the signal, andshapes it in order to provide a processed signal more recognizable. Atthe surface, in the preferred embodiment described here, the processedsignal features a readily recognizable square wave, the high stateportion of which corresponds to the presence of a joint. The signalprocessor 72 includes an amplifier and an analog-to-digital converter(neither shown), which are well-known components. The amplifier enhancesthe signal while the converter is used to convert the analog readingsobtained by the sensor 70 into a more readily recognizable digitalsignal. If desired, the signal processor 72 may incorporate one or morenoise filters of a type known in the art in order to remove noise fromthe signal generated by the sensor 70. Other signal processingtechniques used to enhance the quality of such signals may be applied.

[0061] The detector assembly 66 further includes a data transmitter 74that is operably interconnected with the signal processor 72. The datatransmitter 74 receives the amplified and processed signal created bythe signal processor 72 and transmits it to a distant receiver,typically located at the surface of the wellbore that includes boreholesection 10. The distant receiver might comprise an oscilloscope,computer, or storage medium for the signals.

[0062] In operation, the sensor 70 senses the perturbation provided bythe increased or changed magnetic fields associated with anomalies inthe wall of the casing string, such as the connections or joints betweencasing sections 20, 22 or 20′, 22′. In the case of the collar connection36 shown in FIG. 6, the sensor 70 senses the increased magnetic field inthe surrounding casing resulting from the presence of the externalcollar 38 as well as that provided by the discontinuity 42.

[0063]FIG. 9 illustrates the processing of the signals by the signalprocessor 72. The sensor 70 sends an analog signal 100 to the processor72. As shown, the analog signal 100 is made up of a number of peaks andvalleys that correspond to changes in the magnetic field sensed by thesensor 70. The analog signal 100 includes a reduced baseline signalportion 102 that corresponds to detection by the sensor 70 of continuouscasing walls. The signal 100 also includes an enhanced signal portion104 that corresponds to the detection by the sensor 70 of anomalies,such as discontinuities, holes, or gaps in the surrounding casing walls.The enhanced signal portion 104 is significantly different from thebaseline signal portion 102 due to changes in the borehole magnetic fluxas a result of the discontinuities 32, 34 and gaps 30 present in thecasing 14. As noted, the signal processor 72 contains an amplifier andanalog-to-digital converter, both of which are well-known components.The signal processor 38, therefore, produces a processed digital signal110 based upon the analog signal 100 it receives. The processed signal110 is preferably a square wave that is made up of “high” and “low”states, each of which are indicative of a different condition. This typeof signal is preferred because it provides a more definite indication ofcondition than an analog signal such as signal 100. The high stateportion 112 of the signal 100 is indicative of the presence ofdiscontinuities and/or gaps in the surrounding casing wall and isproduced when the sensor 70 is located adjacent a casing joint, such asjoint 24. Conversely, a low state portion 114 results when there is anabsence of such discontinuities and gaps. The processed signal 110 isreceived by and then transmitted to the surface via the data transmitter74 on a periodic basis, such as every 50 milliseconds.

[0064] As explained, the high state portion 112 of the square wave ofthe processed signal 110 corresponds to the presence of anomalies, suchas discontinuities, gaps, and/or casing mass change in the surroundingcasing wall, while the low state portion 114 of the signal 110 indicatesthe absence of anomalies, such as discontinuities, gaps, and wall masschange, that would affect the surrounding magnetic field. As a result,the length (“x” in FIG. 5) of the high state portion 112 corresponds tothe length of the joint 24 as measured from the upper discontinuity 32to the lower discontinuity 34. The detector assembly 66 permits thedetermination of the length of the joint 24 from the length “x” of thehigh state portion 112 of the signal 110. This capability provides welloperators with greater information regarding the exact location andsizes of joints within a cased wellbore and is, therefore, quitevaluable. More specifically, the detector assembly 66 is typically movedat a relatively constant rate, or velocity, through the wellbore 10.This rate is known and controlled at the surface. When this is the case,the detector assembly 66 is initially positioned at a known depth orlocation in the cased wellbore. If the initial position for the detectorassembly 66 is at the surface of the well, the initial position will beat zero (0) feet. As the detector assembly 66 is lowered though thewellbore, the signal 110 is normally transmitted to the surface, orupdated, in a periodic fashion over time, i. e., every 50 milliseconds.Because the velocity of movement (v) of the detector assembly 66 throughthe cased wellbore is known and the time (t) of the signals is known,the location of the detector assembly 66 within the well can bedetermined. Further, the location of the detector assembly 66 can bereferenced to the presence of a joint between casing sections so thatthe location of these joints is easily tracked and determined.Additionally, the signal length (“x”), discussed earlier, can be easilycorrelated to the length of a certain joint so that the actual length ofthe joint as well as the exact depth of portions of the joint can bedetermined.

[0065] The example is further illustrated in FIG. 9 where the analogsignal 100 changes from its baseline signal 102 to the enhanced signal104 upon encountering the leading portion of a joint, such as the upperdiscontinuity 32 of joint 24. The signal processor 72 converts theanalog signal to the processed signal 110 and, when the leading portionof the joint 24 is detected by the sensor 70, the processed signal 110changes from a low state signal 114 to a high state signal 112 at point116. At the surface, point 116 is correlated with a depth marker (e.g.,222.5 ft.), as calculated by the velocity vs. time relationshipdescribed earlier. Such correlation can be easily accomplished usingknown software to make the appropriate calculations. As the sensor 70 ismoved downwardly and past the lower portion of the joint 24, theprocessed signal 110 changes from a high state signal 112 to a low statesignal 114 at point 118. Point 118 is also correlated with a depthmarker (e.g., 228 ft.). The difference between points 116 and 118 yields5.5 ft., the length of the joint 24.

[0066] Operation of the detector assembly 66 in an exemplary wellbore isillustrated by the sequence of FIGS. 2-4 that show the detector assembly66 being lowered through the cased borehole section 10. In FIG. 2, thedetector assembly 66 is located within the cased borehole section 10 andmoved in the direction of arrow 80 until the sensor 70 is substantiallyadjacent the discontinuity 32 at the upper end of the joint 24. At thispoint, the discontinuity 32 is detected by the sensor 70 and a processedsignal 110 is moved to a high state 112 from a low state 114.

[0067] In FIG. 3, the sensor 70 is located proximate the gaps 30 of thejoint 24. As a result, the processed signal 110 will be maintained inthe high state 112 due to the alteration in the surrounding magneticfield caused by gaps 30. In FIG. 4, the detector assembly 66 has moveddownwardly to the point where the sensor 70 is disposed below the lowerdiscontinuity 34 and is adjacent the wall of the lower casing section22. Due to the absence of gaps 30 or discontinuities 32, 34, theprocessed signal 110 will return to the low state 114.

[0068] It should be appreciated that the detector assembly 66 of thepresent invention may be used to detect any anomaly in the wall of thecasing string. For example, detector assembly 66 is useful in detectingbreaks or ruptures in the wall of the casing 14. Such breaks andruptures result in a change, or perturbation, in the induced magneticfields in the wall of the casing string. As a result, the detectorassembly 66 may be used to find damage in a wellbore casing. The methodsof detecting such damage are substantially the same as those describedabove with respect to detecting casing joints.

[0069] Referring now to FIGS. 10-12, there is shown another applicationof the detector assembly 66 to locate perforations in the casing. Thedetector assembly 66 is the same as that discussed above. Instead oflocating casing joints or casing collars, however, the detector assembly66 operates in the same manner to detect perforations 800, 802 in thewall of the casing 20. Perforations 800, 802 are small, generally lessthan one inch in diameter and typically having a diameter of 0.25inches. Thus, perforations 800, 802 have the same magnetic forcequalities as gaps, such as air gaps 30 shown in FIG. 4. Due to thenatural magnetic field of the casing 14, the perforations produce fringeeffects 58 due to the lines of attractive magnetic forces 60 across thesides of the perforations 800, 802. The attractive magnetic forces 60produce an increased magnetic signature 63 just as with the casingjoints and casing collars discussed above. With a detector assembly 66having a resolution high enough to detect the increased magneticsignatures of the perforations 800, 802, the exact location of theperforations can be determined. The detection of perforations issensitive to the location of the perforations 800, 802 and the axiallocation of the detector assembly 66 in the casing 20.

[0070] For perforation patterns with perforations 800, 802 on one sideof the casing 20 in a given plane perpendicular to the longitudinal axisof the casing 20 as shown in FIG. 10, only one sensor 70 is needed. Forperforation patterns with perforations 800, 802 having a spiral patternwith spiraling rows 804 as shown in FIG. 11, only one sensor may beneeded since the perforations are staggered. For perforation patternswith opposing perforations 800, 802 on more than one side of the casing20 per plane as shown in FIG. 12, more than one sensor 70, such assensors 70 a, 70 b, is needed to detect individual opposed perforations.However, with perforation patterns with perforations 800, 802 on morethan one side per plane, one sensor 70 may still be used to detect theperforation zone of the casing 20 because it is not necessary to detectthe individual perforations 800, 802.

[0071] In operation, the detector assembly 66 may be included as part ofa bottom hole assembly (BHA) 76 on a workstring 82 when the BHA 76includes nozzles or ports 78 and one or more packers 80 a, 80 b. Thedetector assembly 66 typically has a resolution of approximately 0.1inch so that it can detect the increased magnetic signature caused bythe fringe effects of an anomaly of 0.1 inch or more such as aperforation of approximately 0.25 inch diameter. The BHA 76 is loweredinto the casing bore 16 to a depth such that ports 78 are below theregion of perforations 800, 802 in casing 20. The BHA 76 is then raiseduntil the detector assembly 66 senses the increased magnetic signatureassociated with the perforations 800, 802. When the detector assembly 66locates perforated region in the casing 20, the BHA 76 is raised a knowndistance such that the ports are substantially aligned with one or moreperforations 800, 802. Likewise, the BHA 76 is also raised until thepackers 80 a, 80 b straddle the perforated zone in a blank section ofcasing (a section free of perforations and casing joints). Once theposition is confirmed, the packers are set and the well operationcommences. The well operation may include any well stimulation ortreatment including fracing, acid treatment, or other operation toenhance production.

[0072] Referring now to FIG. 13, there is shown another preferredapplication of the detector assembly 66 of the present invention. Thedetector assembly 66 is substantially the same as that previouslydescribed. In the preferred method illustrated in FIG. 13, the detectorassembly 66 is lowered through a cased borehole with casing 14. Thecasing 14 is cemented by cement 18 into the borehole extending throughformation 12. The casing 14 includes a lower terminal end 106 with theearthen borehole 108 extending below the terminal end 106 of casing 14.During well operations, it may be necessary to locate the lower terminalend 106 of casing 14 so that well operations may be conducted in thatportion of borehole 108 extending below casing 14. One such welloperation includes drilling a borehole below casing 14 that has adiameter equal to or greater than the outer diameter of casing 14 fordisposing additional casing of a similar diameter below casing 14. Thedetector assembly 66 with sensor 70 passes through borehole 108 to sensethe change in the natural magnetic field between casing 14 and theearthen borehole wall of borehole 108 extending below terminal end 106.Not only will there be a reduced magnetic field below terminal end 106,but fringe effects at the terminal end 106 produce perturbations in themagnetic field that sensor 70 detects. The magnetic signature producedby lower terminal end 106 permits the accurate location of terminal end106 and, in particular, the upper end of the earthen borehole 108extending below casing 14 for conducting of well operations.

[0073] While preferred and alternative embodiments have been shown anddescribed, modifications can be made by one skilled in the art withoutdeparting from the spirit or teaching of this invention. The embodimentsas described are exemplary only and are not limiting. Many variationsand modifications of the system and apparatus are possible and arewithin the scope of the invention. Accordingly, the scope of protectionis not limited to the embodiments described, but is only limited by theclaims that follow, the scope of which shall include all equivalents ofthe subject matter of the claims.

What is claimed is:
 1. An apparatus for detecting an anomaly in theuniformity of the wall of a string of casing extending into a well, thestring of casing having a naturally induced magnetic field and theanomaly causing a fringe effect associated with the naturally inducedmagnetic field, comprising: a magnetoresistive field sensor adapted tobe suspended into the casing; and said sensor producing a first signalwhile passing through a uniform casing wall and a second signal whilepassing the fringe effect of the anomaly.
 2. The apparatus of claim 1wherein the sensor produces said second signal while passing an anomalyhaving dimensions of less than one inch.
 3. The apparatus of claim 1wherein the sensor produces said second signal while passing an anomalyin the form of a gap between the terminal ends of two adjacent casings.4. The apparatus of claim 1 wherein the sensor produces said secondsignal while passing an anomaly in the form of an enlarged thickness inthe casing string.
 5. The apparatus of claim 1 wherein the sensorproduces said second signal while passing an anomaly in the form of airgaps in threads in the joints connecting the casings.
 6. The apparatusof claim 1 wherein the sensor produces said second signal while passingan anomaly in the form of a hole through the wall of the casing.
 7. Theapparatus of claim 1 wherein the sensor produces said second signalwhile passing an anomaly in the form of a perforation in the casingwall.
 8. The apparatus of claim 1 wherein the sensor produces saidsecond signal while passing an anomaly in the form of damage to the wallof the casing string.
 9. The apparatus of claim 1 wherein the sensorproduces said second signal while passing an anomaly in the form of aregion substantially free of the magnetic field of the casing string.10. The apparatus of 1 wherein said sensor includes magnetic andnon-magnetic layers of materials.
 11. The apparatus of claim 1 furthercomprising a signal processor operably interconnected with said sensorto generate a processed signal indicative of the anomaly.
 12. Theapparatus of claim 11 further comprising a means for receiving a signalgenerated by the signal generator and transmitting it to a remotelocation.
 13. The apparatus of claim 12 wherein the signal processorcomprises an amplifier.
 14. The apparatus of claim 12 wherein the signalprocessor comprises an analog-to-digital converter.
 15. The apparatus ofclaim 12 wherein the signal processor comprises a noise filter.
 16. Theapparatus of claim 1 further comprising a pressure barrel housing thatsubstantially encloses the magnetoresistive field sensor.
 17. Theapparatus of claim 16 wherein the pressure barrel housing issubstantially comprised of a non-magnetic material.
 18. The apparatus ofclaim 17 wherein the non-magnetic material comprises beryllium copper.19. A well tool for detecting an anomaly in the uniformity of the wallof a string of casing extending into a well, t he string of casinghaving a naturally induced magnetic field and the anomaly causing afringe effect associated with the naturally induced magnetic field,comprising: a pressure barrel; and a sensor disposed within saidpressure barrel and adapted to generate a signal when exposed to thenaturally induced magnetic signature field that is caused by the fringeeffects associated with the anomaly, said sensor being itselfsubstantially free of a magnetic signature.
 20. The well tool of claim19 further comprising a signal processor associated with said sensor,said signal processor configured to convert said signal into digitaldata.
 21. The well tool of claim 19 wherein said sensor is a giantmagnetoresistive field sensor operatively connected to a workstringadapted to convey signals, said sensor being adapted to transmit asignal through said workstring when exposed to the fringe effectassociated with the naturally induced magnetic field.
 22. The well toolof claim 21 wherein the sensor has a resolution of approximately 0.1inch.
 23. A method of locating an anomaly in a tubular member in a well,the tubular member having a naturally induced magnetic field,comprising: passing a sensor through the tubular member; sensing ananomaly causing a fringe effect associated with the naturally inducedmagnetic field; producing a signal from the sensor indicative of theanomaly.
 24. The method of claim 23 wherein the anomaly is a perforationin the tubular member.
 25. The method of claim 24 wherein the anomaly isa void associated with the perforation.
 26. The method of claim 24wherein the void is a region substantially free of the magnetic fieldsof the tubular member.
 27. The method of claim 23 wherein the sensorminimizes any magnetic field associated with the sensor.
 28. The methodof claim 23 wherein the anomaly is detected by positioning the sensorproximate the anomaly, the sensor being configured to detect the regionsubstantially free of the naturally induced magnetic fields produced bythe anomaly.
 29. The method of claim 23 wherein the sensor is positionedproximate the anomaly by disposing it through a tubular member using aworkstring that is adapted to transmit electrical power.
 30. The methodof claim 23 further comprising amplifying the signal.
 31. The method ofclaim 23 further comprising converting the signal to a processed digitalsignal.
 32. The method of claim 31 wherein the processed digital signalcomprises a square wave.
 33. The method of claim 30 further comprisingtransmitting the signal to a remote receiver.
 34. A method of detectingone or more perforations in the wall of a tubular member having a firstmagnetic signature being attributable to a naturally induced magneticflux, comprising: disposing a magnetoresistive sensor within a tubularmember; generating a signal from the sensor corresponding to the firstmagnetic signature; moving the sensor proximate a perforation in thewall of the tubular member; and causing a change in the signalindicative of the presence of the perforation, the change beingattributable to fringe effects associated with the perforation.
 35. Themethod of claim 34 further comprising maintaining a magnetic-free areaproximate to the sensor.
 36. A method of detecting one or moreperforations in the wall of a tubular member having a first magneticsignature being attributable to a naturally induced magnetic flux,comprising: disposing a magnetoresistive sensor within a tubular member;generating a signal from the sensor corresponding to the first magneticsignature; moving the sensor proximate a perforation in the wall of thetubular member; and causing a change in the signal indicative of thepresence of the perforation, the change being attributable to a fringeeffect associated with the perforation.
 37. The method of claim 36further comprising processing the signal to obtain a more recognizableprocessed signal.
 38. A method of detecting an increase in an amplitudeof a magnetic field associated with fringe effects in a naturallyinduced magnetic field within a string of tubular members, the methodcomprising: disposing a sensor proximate a string of tubular membershaving the naturally induced magnetic field; sensing the increase in theamplitude of the naturally induced magnetic field with the sensor, theincrease in the amplitude being caused by a fringe effect indicative ofa perforation in a wall of a tubular member.
 39. The method of claim 38wherein the magnetic field in the string of tubular members is inducedby the earth's natural magnetic fields.
 40. The method of claim 39further comprising generating a signal indicative of the increase in theamplitude of the induced field.
 41. The method of claim 40 furthercomprising transmitting the signal to a remote location.
 42. A method ofdetecting perturbations in a naturally induced magnetic field within astring of tubular members, the method comprising: disposing a sensorproximate a string of tubular members having the naturally inducedmagnetic field; and sensing a perturbation in the naturally inducedmagnetic field with the sensor, the perturbation being caused by afringe effect indicative of a perforation in a wall of a tubular member.43. A method of detecting casing joints in a naturally induced magneticfield within a string of tubular members, the method comprising:disposing a sensor proximate a string of tubular members having thenaturally induced magnetic field; sensing a change in the magnitude ofthe magnetic field indicative of the presence of a casing jointconnecting adjacent tubular members.
 44. A method of detectingperturbations in a naturally induced magnetic field within a string oftubular members, the method comprising: disposing a sensor proximate astring of tubular members having the naturally induced magnetic field;sensing a perturbation in the naturally induced magnetic field with thesensor, the perturbation being caused by a fringe effect indicative of aperforation in a wall of a tubular member; and negating any magneticfield associated with the sensor.
 45. The apparatus of claim 1 whereinthe sensor produces the second signal while passing an anomaly in theform of a terminal end of the string of casing as the sensor passes intoan uncased section of the well.
 46. A method of detecting perturbationsin a naturally induced magnetic field within a string of tubularmembers, the method comprising: disposing a sensor proximate a string oftubular members having the naturally induced magnetic field; sensing aperturbation in the naturally induced magnetic field with the sensor,the perturbation being caused by a fringe effect indicative of aterminal end of the string of tubular members; and negating any magneticfield associated with the sensor.